고분자 전해질 연료전지에서 백금 촉매의 용해 및 이동 메커니즘에 관한 연구

Abstract

The distribution patterns of Pt crystals that have moved from electrodes to the membrane phase of membrane electrode assembly (MEA) are monitored using transmission electron microscopy (TEM) after long-term operation of a polymer electrolyte membrane fuel cell (PEMFC) at various operating and feed conditions. The dissolution of cathode Pt and subsequent migration into the membrane is readily observed when residual oxygen concentration inside the cathode are kept high under low current density conditions. Dissolution of anodic Pt can also be observed under constant-current operation when hydrogen feed is kept low to induce a hydrogen shortage on the Pt surface. It is postulated that the Pt at the both electrodes can be dissolved by chemical and electrochemical oxidation to PtO in the presence of residual oxygen. The Pt ions that are dissolved in water migrate into the membrane phase and undergo repeated oxidation/dissolution and reduction/deposition by crossover of oxygen and hydrogen, respectively. As a result, the distribution patterns and crystal sizes of the migrated Pt are strongly dependent on the relative concentrations of the crossover oxygen and hydrogen. The final position of the deposited Pt band is located in the region where crossover oxygen becomes depleted, typically between 1 and 10？m away from the cathode？membrane interface. Higher concentrations of oxygen and hydrogen in the membrane yield sharper and narrower Pt bands with large Pt aggregates, whereas lower concentrations yield wider distribution bands with smaller Pt crystals.It is also proposed possible anode flooding to explain the distribution of migrated Pt after MEA operation under low stoichiometric ratio (SR) hydrogen condition. The low hydrogen flow rate can cause flooding in anode by accumulated water. Due to the flooded water the oxygen diffuses into the hydrogen filled anode. This can cause irreversible damages on both electrodes. On various MEA operations under low SR hydrogen condition, platinum dissolution in both electrodes and cathode carbon corrosion rate increase incredibly. Besides, fuel depletion give severe damage on anode carbon showing the reversed cell potential.